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When displaying a Web Mapping Application in Internet Explorer 6 that contains a cached map service in PNG24 format, the performance of rendering the map image is slower than when:
1) The same application is used in an Internet Explorer 7 or Firefox browser.
2) The service is cached in PNG8 or PNG32 format and used in Internet Explorer 6.
The ArcGIS Server .NET Web Application Developer Framework (ADF) is designed to display the map cache tiles directly in the browser when possible, so that the optimal performance is achieved.
Due to a limitation with an Internet Explorer 6 browser's ability to display transparency for PNG24 images, the Web ADF must do extra processing to show the transparency for PNG24 images. The Web ADF is designed to detect when an application meets the following criteria:
1) Contains a PNG24 cached service on top of another service.
2) Is displayed in an Internet Explorer 6 browser.
When this scenario is encountered, the cache tiles from the service in PNG24 format are not displayed directly in the browser. Instead, a request is made for a new image back to the GIS Server, where the necessary transparency can be applied.
This alternative method results in the difference in performance.